Fiber reinforced composites comprise a fibrous or filamentary material embedded in a matrix. The fiber is the load bearing component and the matrix dissipates loads to the fibers, maintains fiber orientation, and protects the fiber from damaging environmental conditions.
Composites are classified according to their matrix phase. For example, there are polymer matrix composites (PMCs), ceramic matrix composites (CMCs), and metal matrix composites (MMCs). Other types of composites are also known, such as glass matrix composites, carbon matrix composites, etc.
Polymer matrix composites utilize a resin system along with a catalyst or curing agent and fiber reinforcement. These materials are used in consumer goods such as boats, piping, auto bodies, etc., as well as in the production of a wide spectrum of industrial components.
High strength, high modulus composite materials are often termed "advanced composites." Low weight, high strength, high stiffness materials are especially attractive for aircraft and aerospace structural components. Advanced composites have also been adopted for use in the manufacture of sporting goods such as high performance golf, tennis, fishing, and archery equipment. Advanced composites are also finding increased use in the industrial and automotive markets, as well as for other weight sensitive applications.
Fiber reinforced resin composites are typically formed by laying up a plurality of plies formed of reinforcing fibers. The plies may be preimpregnated with resin in an uncured or partially cured state. The resin is cured by applying heat and usually pressure, although catalytic curing at low temperature is also known, e.g., curing at room temperature using a catalyst-promoter system. Alternatively, resin layers may be added to stacks of dry plies and the resin infused into the dry plies as it is cured.
Due to the ply-by-ply nature of resin composites, the susceptibility to delamination along interlaminar planes is an intrinsic and severe problem of these materials [1]. Interlaminar stresses due to mismatch of anisotropic mechanical and thermal properties of plies occur at free edges, joints, matrix cracks, and under out-of-plane loading. Delamination is often the dominating failure mode in laminates subjected to impact and fatigue loading.
Idealized models of edge delamination exhibit singular concentrations of interlaminar shear and peel stresses near edges [2]. High interlaminar stresses occur in the vicinity of cracks in primary reinforcing plies. This is especially dangerous because interlaminar stresses act on an unreinforced plane. Such a plane is always present between the plies with different fiber orientations.
A number of methods to prevent delamination have been developed over the years [3, 4]. These include matrix toughening, optimization of stacking sequence, laminate stitching, braiding, edge cap reinforcement, critical ply termination, and replacement of a stiff ply by one that has softer regions. Most designs to reduce delamination resulted in significant cost or weight penalties.
One attempt to improve delamination resistance involves the modification of the chemistry of the resin composites to increase resin fracture toughness while maintaining composite compression strength. Most methods of increasing the fracture toughness of a brittle resin, such as an epoxy, involve the addition of at least one component with a lower shear modulus than the resin base. This approach is disadvantageous in that it reduces the overall shear modulus, often below that necessary to maintain composite compression performance, and increases the susceptibility of the composite to heat and solvents.
Ductile interleaving [5] is an effective method to improve delamination resistance. Layers of toughened resins (interleaves) are inserted at interfaces between the primary plies in this method. Rubber or thermoplastic particle toughening is often utilized. Substantial improvement in interlaminar fracture toughness was achieved by this method. However, the thickness of the toughened interleaves is usually comparable to the thickness of the primary reinforcing plies. Accordingly, this method is generally only suitable for use at critical interfaces. The use of these interleaves at multiple interfaces may substantially increase weight and/or reduce in-plane properties of laminates.
Another approach to prevent delamination of fiber reinforced composite materials has been to add more mass to keep the stresses low. This approach, however, is disadvantageous in that it substantially reduces the weight savings advantage associated with fiber reinforced resin composites.
Another approach used to solve the delamination problem is to design discrete portions of a composite to have a substantially higher stiffness than the remainder of the composite. The stiffer portions become the primary load carrying paths and can be varied to meet structural requirements. The disadvantage of this approach is that it places severe restrictions on design. Also, weight is increased by the material added to discretize the stiffness of various load paths. The end result is a structure that is heavier and more difficult to manufacture than composite structures that do not have these design constraints.
Delamination resistance of fiber reinforced resin composites can be improved by cross-ply stitching laminate plies together. While this method does not substantially increase weight, it increases production costs. Also, this technique may be difficult to implement in composites structures having complicated structural configurations.
Also, the interlaminar fracture toughness of composites employing a high temperature, e.g., polyimide, matrix material is generally superior to those employing a brittle thermoset resin matrix material, e.g., epoxy. However, there is experimental evidence for interface degradation and delamination due to matrix microcracking after thermal aging.
Reinforcement of interfaces in laminates by fibers of conventional diameters has been experimentally explored [6]. Random mats of commercial polyaramid, polyester, and glass fibers were studied. Improvement in the Mode I critical energy release rate was reported for all in-lay materials. Best results were obtained with polyaramid fibers combined with a toughened epoxy adhesive. However, the thickness of the inner, randomly reinforced layer was equal or higher than the thickness of primary reinforcing plies. Further, fiber mats required preimpregnation. The toughened adhesive layer required additional curing, leading to some embrittlement of the primary plies. Structurally, thick interlayers reinforced by fibers of conventional diameters can be regarded as additional plies rather than interface modifiers. High thickness of these layers and low volume fraction of randomly distributed fibers lead to substantial reduction of in-plane properties, such as compression strength, of laminates.
It would, therefore, be desirable to provide improved composites providing better damage tolerance due to delamination resistance and microcrack arresting properties.
It would also be desirable to provide a technique for imparting delamination resistance and microcrack arresting capability to a composite material that is inexpensive, does not require a significant increase in weight or volume of the resulting composite, and requires only minimal effect on current laminating techniques and facilities.